Reactions of H₂, HD, and D₂ with H₂⁺, HD⁺, and D₂⁺: Product-Channel Branching Ratios and Simple Models

We present measurements of the product-channel branching ratios of the reactions (i) HD⁺ + HD forming H₂D⁺ + D (38.1(30)%) and HD₂⁺ + H (61.9(30)%), (ii) HD⁺ + D₂ forming HD₂⁺ + D (61.4(35)%) and D₃⁺ + H (38.6(35)%), and (iii) D₂⁺ + HD forming HD₂⁺+ D (60.5(20)%) and D₃⁺ + H (39.5(20)%) at collision energies E_coll near zero, i.e., below k_B × 1 K. These branching ratios are compared with branching ratios predicted using three simple models: a combinatorial model (M1), a model (M2) describing the reactions as H-, H⁺-, D-, and D⁺-transfer processes, and a statistical model (M3) that relates the reaction rate coefficients to the translational and rovibrational state densities of the HD_3-⁺ + H/D (n = 0, 1, 2 or 3) product channels. The experimental data are incompatible with the predictions of models M1 and M2 and reveal that the branching ratios exhibit clear correlations with the product state densities.